In javascript anywhere you see await, you've introduced an explicit scheduling point. There are cases where even in a single threaded env you want to "wait" until some other async process is complete. To use one of the old school examples:

    function transfer(amount, acct1, acct2) {
        var current_balance = await acc1.balance()
        if current_balance > amount {
            await acct1.sub(amount)
            await acct2.add(amount)
        }
    }

Now what happens if you get multiple calls to transfer? What you want is probably something like:

    function transfer(amount, acct1, acct2) {
         await acct1.Lock()
         ....
         await acct1.UnLock()
    }